SYNOPSIS

DESCRIPTION

This is an implementation of the SCTP protocol as defined in RFC2960
and RFC3309. It is a message oriented, reliable transport protocol with
direct support for multihoming that runs on top of ip(7), and supports
both v4 and v6 versions.
Like TCP, SCTP provides reliable, connection oriented data delivery
with congestion control. Unlike TCP, SCTP also provides message
boundary preservation, ordered and unordered message delivery, multi-
streaming and multi-homing. Detection of data corruption, loss of data
and duplication of data is achieved by using checksums and sequence
numbers. A selective retransmission mechanism is applied to correct
loss or corruption of data.
This implementation supports a mapping of SCTP into sockets API as
defined in the draft-ietf-tsvwg-sctpsocket-10.txt(Sockets API
extensions for SCTP). Two styles of interfaces are supported.
A one-to-many style interface with 1 to MANY relationship between
socket and associations where the outbound association setup is
implicit. The syntax of a one-to-many style socket() call is
sd=socket(PF_INET,SOCK_SEQPACKET,IPPROTO_SCTP);
A typical server in this style uses the following socket calls in
sequence to prepare an endpoint for servicing requests.
1. socket()
2. bind()
3. listen()
4. recvmsg()
5. sendmsg()
6. close()
A typical client uses the following calls in sequence to setup an
association with a server to request services.
1. socket()
2. sendmsg()
3. recvmsg()
4. close()
A one-to-onestyle interface with a 1 to 1 relationship between socket
and association which enables existing TCP applications to be ported to
SCTP with very little effort. The syntax of a one-to-one style socket()
call is
sd=socket(PF_INET,SOCK_STREAM,IPPROTO_SCTP);
A typical server in one-to-one style uses the following system call
sequence to prepare an SCTP endpoint for servicing requests:
1. socket()
2. bind()
3. listen()
4. accept()
The accept() call blocks until a new association is set up. It returns
with a new socket descriptor. The server then uses the new socket
descriptor to communicate with the client, using recv() and send()
calls to get requests and send back responses. Then it calls
5. close()
to terminate the association. A typical client uses the following
system call sequence to setup an association with a server to request
services:
1. socket()
2. connect()
After returning from connect(), the client uses send() and recv() calls
to send out requests and receive responses from the server. The client
calls
3. close()
to terminate this association when done.

ADDRESSFORMATS

SCTP is built on top of IP (see ip(7)). The address formats defined by
ip(7) apply to SCTP. SCTP only supports point-to-point communication;
broadcasting and multicasting are not supported.

SYSCTLS

These variables can be accessed by the /proc/sys/net/sctp/* files or
with the sysctl(2) interface. In addition, most IP sysctls also apply
to SCTP. See ip(7).
addip_enable
Enable SCTP ADDIP(Dynamic Address Reconfiguration) Support. This
is off by default.
association_max_retrans
Maximum number of consecutive retransmissions to a peer before
an endpoint considers that the peer is unreachable and closes
the association. The default value is 10.
cookie_preserve_enable
Handle COOKIE PRESERVATIVE parameter in the INIT chunk. This is
on by default.
hb_interval
This is the interval when a HEARTBEAT chunk is sent to a
destination transport address to monitor the reachability of an
idle destination transport address. The default is 30 seconds
and is maintained in msecs.
max_burst
Maximum number of new data packets that can be sent in a burst.
The default value is 4.
max_init_retransmits
Maximum number of times an INIT chunk or a COOKIE ECHO chunk is
retransmitted before an endpoint aborts the initialization
process and closes the association. The default value is 8.
path_max_retrans
Maximum number of consecutive retransmissions over a destination
transport address of a peer endpoint before it is marked as
inactive. The default value is 5.
prsctp_enable
Enable PR-SCTP. This is on by default.
rcvbuf_policy
This controls the socket receive buffer accounting policy. The
default value is 0 and indicates that all the associations
belonging to a socket share the same receive buffer space. When
set to 1, each association will have its own receive buffer
space.
rto_alpha_exp_divisor
This is the RTO.Alpha value when expressed in right shifts and
is used in RTO calculations. The default value is 3.
rto_beta_exp_divisor
This is the RTO.Beta value when expressed in right shifts and is
used in RTO calculations. The default value is 2.
rto_initial
This is the initial value of RTO(retransmission timeout) that is
used in RTO calculations. The default value is 3 seconds and is
maintained in msecs.
rto_max
This is the maximum value of RTO(retransmission timeout) that is
used in RTO calculations. The default value is 60 seconds and is
maintained in msecs.
rto_min
This is the minimum value of RTO(retransmission timeout) that is
used in RTO calculations. The default value is 1 second and is
maintained in msecs.
sack_timeout
Delayed SACK timeout. The default value is 200msecs.
sndbuf_policy
This controls the socket sendbuffer accounting policy. The
default value is 0 and indicates that all the associations
belonging to a socket share the same send buffer space. When set
to 1, each association will have its own send buffer space.
valid_cookie_life
This is the maximum lifespan of the Cookie sent in an INIT ACK
chunk. The default value is 60 secs and is maintained in msecs.

STATISTICS

These variables can be accessed by the /proc/net/sctp/* files.
assocs Displays the following information about the active
associations. assoc ptr, sock ptr, socket style, sock state,
association state, hash bucket, association id, bytes in
transmit queue, bytes in receive queue, user id, inode, local
port, remote port, local addresses and remote addresses.
eps Displays the following information about the active endpoints.
endpoint ptr, sock ptr, socket style, sock state, hash bucket,
local port, user id, inode and local addresses.
snmp Displays the following statistics related to SCTP states,
packets and chunks.
SctpCurrEstab
The number of associations for which the current state is
either ESTABLISHED, SHUTDOWN-RECEIVED or SHUTDOWN-
PENDING.
SctpActiveEstabs
The number of times that associations have made a direct
transition to the ESTABLISHED state from the COOKIE-
ECHOED state. The upper layer initiated the association
attempt.
SctpPassiveEstabs
The number of times that associations have made a direct
transition to the ESTABLISHED state from the CLOSED
state. The remote endpoint initiated the association
attempt.
SctpAborteds
The number of times that associations have made a direct
transition to the CLOSED state from any state using the
primitive ’ABORT’. Ungraceful termination of the
association.
SctpShutdowns
The number of times that associations have made a direct
transition to the CLOSED state from either the SHUTDOWN-
SENT state or the SHUTDOWN-ACK-SENT state. Graceful
termination of the association.
SctpOutOfBlues
The number of out of the blue packets received by the
host. An out of the blue packet is an SCTP packet
correctly formed, including the proper checksum, but for
which the receiver was unable to identify an appropriate
association.
SctpChecksumErrors
The number of SCTP packets received with an invalid
checksum.
SctpOutCtrlChunks
The number of SCTP control chunks sent (retransmissions
are not included). Control chunks are those chunks
different from DATA.
SctpOutOrderChunks
The number of SCTP ordered data chunks sent
(retransmissions are not included).
SctpOutUnorderChunks
The number of SCTP unordered chunks(data chunks in which
the U bit is set to 1) sent (retransmissions are not
included).
SctpInCtrlChunks
The number of SCTP control chunks received (no duplicate
chunks included).
SctpInOrderChunks
The number of SCTP ordered data chunks received (no
duplicate chunks included).
SctpInUnorderChunks
The number of SCTP unordered chunks(data chunks in which
the U bit is set to 1) received (no duplicate chunks
included).
SctpFragUsrMsgs
The number of user messages that have to be fragmented
because of the MTU.
SctpReasmUsrMsgs
The number of user messages reassembled, after conversion
into DATA chunks.
SctpOutSCTPPacks
The number of SCTP packets sent. Retransmitted DATA
chunks are included.
SctpInSCTPPacks
The number of SCTP packets received. Duplicates are
included.

SOCKETOPTIONS

To set or get a SCTP socket option, call getsockopt(2) to read
or setsockopt(2) to write the option with the option level
argument set to SOL_SCTP.SCTP_RTOINFO.
This option is used to get or set the protocol parameters
used to initialize and bound retransmission timout(RTO).
The structure sctp_rtoinfo defined in
/usr/include/netinet/sctp.h is used to access and modify
these parameters.
SCTP_ASSOCINFO
This option is used to both examine and set various
association and endpoint parameters. The sturcture
sctp_assocparams defined in /usr/include/netinet/sctp.h
is used to access and modify these parameters.
SCTP_INITMSG
This option is used to get or set the protocol parameters
for the default association initialization. The structure
sctp_initmsg defined in /usr/include/netinet/sctp.h is
used to access and modify these parameters.
Setting initialization parameters is effective only on an
unconnected socket (for one-to-many style sockets only
future associations are effected by the change). With
one-to-one style sockets, this option is inherited by
sockets derived from a listener socket.
SCTP_NODELAY
Turn on/off any Nagle-like algorithm. This means that
packets are generally sent as soon as possible and no
unnecessary delays are introduced, at the cost of more
packets in the network. Expects an integer boolean flag.
SCTP_AUTOCLOSE
This socket option is applicable to the one-to-many style
socket only. When set it will cause associations that are
idle for more than the specified number of seconds to
automatically close. An association being idle is defined
an association that has NOT sent or received user data.
The special value of 0 indicates that no automatic close
of any associations should be performed. The option
expects an integer defining the number of seconds of idle
time before an association is closed.
SCTP_SET_PEER_PRIMARY_ADDR
Requests that the peer mark the enclosed address as the
association primary. The enclosed address must be one of
the association’s locally bound addresses. The structure
sctp_setpeerprim defined in /usr/include/netinet/sctp.h
is used to make a set peer primary request.
SCTP_PRIMARY_ADDR
Requests that the local SCTP stack use the enclosed peer
address as the association primary. The enclosed address
must be one of the association peer’s addresses. The
structure sctp_prim defined in
/usr/include/netinet/sctp.h is used to make a get/set
primary request.
SCTP_DISABLE_FRAGMENTS
This option is a on/off flag and is passed an integer
where a non-zero is on and a zero is off. If enabled no
SCTP message fragmentation will be performed. Instead if
a message being sent exceeds the current PMTU size, the
message will NOT be sent and an error will be indicated
to the user.
SCTP_PEER_ADDR_PARAMS
Using this option, applications can enable or disable
heartbeats for any peer address of an association, modify
an address’s heartbeat interval, force a heartbeat to be
sent immediately, and adjust the address’s maximum number
of retransmissions sent before an address is considered
unreachable. The structure sctp_paddrparams defined in
/usr/include/netinet/sctp.h is used to access and modify
an address’s parameters.
SCTP_DEFAULT_SEND_PARAM
Applications that wish to use the sendto() system call
may wish to specify a default set of parameters that
would normally be supplied through the inclusion of
ancillary data. This socket option allows such an
application to set the default sctp_sndrcvinfo structure.
The application that wishes to use this socket option
simply passes in to this call the sctp_sndrcvinfo
structure defined in /usr/include/netinet/sctp.h. The
input parameters accepted by this call include
sinfo_stream, sinfo_flags, sinfo_ppid, sinfo_context,
sinfo_timetolive. The user must set the sinfo_assoc_id
field to identify the
association to affect if the caller is using the one-to-
many style.
SCTP_EVENTS
This socket option is used to specify various
notifications and ancillary data the user wishes to
receive. The structure sctp_event_subscribe defined in
/usr/include/netinet/sctp.h is used to access or modify
the events of interest to the user.
SCTP_I_WANT_MAPPED_V4_ADDR
This socket option is a boolean flag which turns on or
off mapped V4 addresses. If this option is turned on and
the socket is type PF_INET6, then IPv4 addresses will be
mapped to V6 representation. If this option is turned
off, then no mapping will be done of V4 addresses and a
user will receive both PF_INET6 and PF_INET type
addresses on the socket.
By default this option is turned on and expects an
integer to be passed where non-zero turns on the option
and zero turns off the option.
SCTP_MAXSEG
This socket option specifies the maximum size to put in
any outgoing SCTP DATA chunk. If a message is larger than
this size it will be fragmented by SCTP into the
specified size. Note that the underlying SCTP
implementation may fragment into smaller sized chunks
when the PMTU of the underlying association is smaller
than the value set by the user. The option expects an
integer.
The default value for this option is 0 which indicates
the user is NOT limiting fragmentation and only the PMTU
will effect SCTP’s choice of DATA chunk size.
SCTP_STATUS
Applications can retrieve current status information
about an association, including association state, peer
receiver window size, number of unacked data chunks, and
number of data chunks pending receipt. This information
is read-only. The structure sctp_status defined in
/usr/include/netinet/sctp.h is used to access this
information.
SCTP_GET_PEER_ADDR_INFO
Applications can retrieve information about a specific
peer address of an association, including its
reachability state, congestion window, and retransmission
timer values. This information is read-only. The
structure sctp_paddr_info defined in
/usr/include/netinet/sctp.h is used to access this
information.